1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to a method for defining physical channel transmit/receiving timings and resource allocation in a Time Division Duplex (TDD) communication system supporting carrier aggregation.
2. Description of the Related Art
The mobile communication system has evolved into a high-speed, high-quality wireless packet data communication system (such as 3rd Generation Partnership Project (3GPP) High Speed Packet Access (HSPA) and Long Term Evolution (LTE), 3GPP2 High Rate Packet Data (HRPD), Ultra Mobile Broadband (UMB), and Institute of Electrical and Electronics Engineers (IEEE) 802.16e standard systems) to provide data and multimedia services beyond voice-oriented services.
As a representative broadband radio communication standard, LTE adopts Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink. Such a multiple access technique is characterized in that the time-frequency resources carrying data or control information are arranged orthogonally to discriminate among the per-user data and/or control information.
In order to prepare against a decoding failure that occurs at initial transmission, LTE adopts Hybrid Automatic Repeat Request (HARQ) for retransmission of the decoding-failed data on the physical layer.
HARQ is a technique in which, when decoding fails, the receiver sends the transmitter a Negative ACKnowledgement (NACK) such that the transmitter retransmits the decoding-failed data. If the data is decoded successfully, the receiver sends the transmitter an ACKnowledgement (ACK) such that the transmitter sends new data.
One of the important features of the broadband communication system is to support a scalable bandwidth for providing a high speed data service. For example, the Long Term Evolution (LTE) system can support various bandwidths, e.g., 20/15/5/3/1.4 Mhz. The service providers can provide the service on a specific bandwidth selected among the diverse bandwidths. Likewise, there can be various terminals having different LTE capabilities for supporting a minimum 1.4 MHz bandwidth and up to a 20 MHz bandwidth.
Meanwhile, LTE-Advanced (LTE-A) aiming to meet the IMT-Advanced requirements can provide a broadband service at a data rate of up to 100 MHz through carrier aggregation. In order to support the high data rate transmission, the LTE-A system requires a bandwidth wider than that of the LTE system while preserving backward compatibility to the legacy systems for supporting LTE User Equipment (UE). For backward compatibility, system bandwidth of the LTE-A system is divided into a plurality of subbands or Component Carriers (CC) that can be used for transmission/reception of LTE UEs and aggregated for the high data rate transmission of the LTE-A system with the transmission/reception process of the legacy LTE system per component carrier.
Typically, the scheduling information for the data to be transmitted on the component carriers is transmitted to the UE in Downlink Control Information (DCI). The DCI can be defined in various formats, and one of the predefined DCI formats can be used according to whether scheduling information is for uplink or downlink, whether the DCI is compact DCI, whether spatial multiplexing with multiple antennas is applied, and whether the DCI is the power control DCI. For example, the DCI format 1 carrying the control information on the uplink data transmitted without application of Multiple Input Multiple Output (MIMO) can include the following control information.                Resource allocation type 0/1 flag: to differentiate between resource allocation type 0 and resource allocation type 1. Type 0 allocates resources in a unit of Resource Block Groups (RBGs) using a bitmap format. In the LTE/LTE-A system, the scheduling resource unit is a Resource Block (RB) representing time and frequency resource region, and each RBG can be composed of a plurality of RBs. The RBG can be a basic unit of scheduling resources in type 0. In type 1, specific RB can be allocated in the RBG.        Resource block assignment: to indicate resource blocks to be assigned to the UE. The basic unit of radio resource allocation is an RB representing a time and frequency region.        Modulation and coding scheme and redundancy version: to indicate modulation scheme and coding rate used in data transmission.        HARQ process number: to indicate the number of a HARQ process.        New Data Indicator (NDI): to indicate whether the packet is a new transmission or a retransmission.        Redundancy version: to indicate the redundancy version of HARQ.        Transmission Power Control (TPC) command for Physical Uplink Shared Channel (PUSCH): to indicate TPC command for PUSCH.        
The DCI is channel-coded and modulated and then transmitted on a Physical Downlink Control Channel (PDCCH).
FIG. 1 is a diagram illustrating a principle of self-scheduling in an LTE-A system supporting carrier aggregation according to the related art. FIG. 1 is directed to a situation where an evolved Node B (eNB) schedules downlink data of a UE in an LTE-A system operating with two component carriers (e.g., CC#1 and CC#2).
Referring to FIG. 1, the DCI 101 transmitted on the CC#1 109 is formatted as defined in the legacy LTE standard, channel-coded, and then interleaved to generate PDCCH 103. The PDCCH 103 carries the scheduling information 113 about the PDCCH as the data channel allocated to the UE on the CC#1 109. The DCI 105 transmitted on the CC#2 111 is formatted as defined in the legacy LTE standard, channel-coded, and then interleaved to generate PDCCH 107. The PDCCH 107 carries the scheduling information 115 about a Physical Downlink Shared Channel (PDSCH) as the data channel allocated to the UE on the CC#2 111.
In the LTE-A system supporting carrier aggregation, the data and/or DCI for supporting the data transmission can be transmitted per component carrier as shown in FIG. 1. Such a scheduling technique is referred to as self-scheduling. In a case of DCI, however, it can be transmitted on another component carrier different form the component carrier carrying the data, and this is referred to as cross carrier scheduling. In the exemplary case of FIG. 1, when it is difficult to expect high reliability of DCI reception performance due to high interference on the CC#2, the DCI can be transmitted on the CC#1, which is experiencing relatively low interference.
In a case of PDSCH carrying data, it is possible to overcome the interference with frequency selective scheduling or HARQ. In a case of PDCCH carrying DCI, however, HARQ is not applied and it is not possible to apply the frequency selective scheduling due to system band-wide transmission characteristic and thus there is a need of a method for mitigating interference.
FIG. 2 is a diagram illustrating a principle of cross carrier scheduling in an LTE-A system supporting carrier aggregation according to the related art. FIG. 2 is directed to an exemplary cross carrier scheduling for an LTE-A UE operating on two aggregated carriers CC#1 209 and CC#2 219. It is assumed that CC#2 experiences relatively large interference as compared to CC#1 such that, when transmitting DCI as the scheduling information for the data transmission on CC#2, it is difficult to expect satisfactory DCI reception performance.
Referring to FIG. 2, the eNB can transmit the DCI on CC#1 209. In order to support the cross carrier scheduling, the eNB transmits a Carrier Indicator (CI) indicating the component carrier targeted by the DCI along with the DCI indicating the resource allocation information and transmission format of the scheduled data. For example, CI=‘00’ indicates CC#1 209 and, CI=‘01’ indicates CC#2 219.
The eNB combines the DCI 201 indicating resource allocation information and transmission format of the scheduled data 207 and the carrier indicator 202 to generate an extended DCI. The eNB performs channel coding, modulation, and interleaving on the extended DCI to generate a PDCCH 205. Here, the eNB maps the extended DCI to a respective region of the PDCCH 205 of CC#1 209.
The eNB combines the DCI 211 indicating the resource allocation information and transmission format of the data 217 scheduled on CC#2 and the carrier indicator 212 to generate an extended DCI. Next, the eNB maps the extended DCI to a respective region of the PDCCH 205 of CC#1 209.
The Time Division Duplex (TDD) system uses a common frequency for uplink and downlink which are discriminated in the time domain. In LTE and LTE-A TDD systems, the uplink and downlink signals are discriminated by subframe. A radio frame can be divided into equal number of uplink and downlink subframes according to the uplink and downlink traffic load. The number of uplink subframes may be greater than that of the downlink subframes and vice versa. In the LTE system, the subframe has a length of 1 ms, 10 subframes form a radio frame.
TABLE 1Uplink-downlinkSubframe numberconfiguration01234567890DSUUUDSUUU1DSUUDDSUUD2DSUDDDSUDD3DSUUUDDDDD4DSUUDDDDDD5DSUDDDDDDD6DSUUUDSUUD
Table 1 shows TDD configurations (TDD uplink-downlink configurations) defined in the LTE standard. In Table 1, subframe numbers 0 to 9 indicate the indices of subframes constituting one radio frame. Here, ‘D’ denotes a subframe reserved for downlink transmission, ‘U’ denotes a subframe reserved for uplink transmission, and ‘S’ denotes a special subframe.
Downlink Pilot Time Slot (DwPTS) can carry the downlink control information as the normal subframe does. If the DwPTS is long enough according to the configuration state of the special subframe, it is possible to carry the downlink data too. Guard Period (GP) is the interval used for a downlink-to-uplink switch and its length is determined according to the network configuration. Uplink Pilot Time Slot (UpPTS) can be used for transmitting a UE's Sounding Reference Signal (SRS) for uplink channel state estimation and a UE's Random Access Channel (RACH).
In a case of TDD uplink-downlink configuration #6, the eNB can transmit downlink data and/or control information at subframes #0, #5, and #9 and uplink data and/control information at subframes #2, #3, #4, #7, and #8. Here, # indicates number or index. The subframes #1 and #6 as special subframes can be used for transmitting downlink control information and/or downlink data selectively and SRS or RACH in uplink.
Since the downlink or uplink transmission is allowed for specific time duration in the TDD system, the timing relationship among the uplink and downlink physical channels such as control channel for data scheduling, scheduled data channel, and HARQ ACK/NACK channel (HARQ acknowledgement) corresponding to the data channel should be defined.
In LTE and LTE-A TDD systems, the timing relationship between PDSCH and Physical Uplink Control channel (PUCCH) carrying uplink HARQ ACK/NACK corresponding to the PDSCH or PUSCH is as follows.
The UE receives the PDSCH transmitted by the eNB at an (n−k)th subframe and transmits an uplink HARQ ACK/NACK corresponding to the received PDSCH at an nth subframe. Here, k denotes an element of a set K, and K is defined as shown in Table 2.
TABLE 2UL-DLSubframe nConfiguration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7, ——4, 63——7, 6, 116, 55, 4—————4——12, 8, 7, 116, 5, 4, 7 ——————5——13, 12, 9, ———————8, 7, 5, 4, 11, 66——775——77—
FIG. 3 is a diagram illustrating a timing relationship between a PDSCH and an uplink HARQ ACK/NACK in a legacy LTE system operating with TDD uplink-downlink configuration #6 according to the related art. FIG. 3 shows which subframe carries uplink HARQ ACK/NACK corresponding to PDSCH that is transmitted in a downlink subframe or a special subframe in TDD uplink-downlink configuration #6 as defined in Table 2.
For example, the UE transmits, at subframe #7 of ith radio frame, the uplink HARQ ACK/NACK 303 corresponding to the PDSCH 301 transmitted by the eNB at subframe #1 of ith subframe. At this time, the DCI including the scheduling information on the PDSCH 301 is transmitted through a PDCCH of the subframe which also carries the PDSCH. For another example, the UE transmits, at the subframe #4 of (i+1)th radio frame, the uplink HARQ ACK/NACK 307 corresponding to PDSCH 305 transmitted by the eNB at subframe #9 of the ith radio frame. Likewise, the DCI including the scheduling information on the PDSCH 305 is transmitted through the PDCCH of the subframe which also carries PDSCH.
The LTE and LTE-A systems adopt an asynchronous HARQ in the downlink in which the data retransmission timing is not fixed. That is, when an HARQ ACK fed back by the UE in response to the HARQ initial transmission data transmitted by the eNB is received, the eNB determines the next HARQ retransmission timing freely according to the scheduling operation. The UE buffers the data that failed in decoding for a HARQ operation and combines the buffered data with the next HARQ retransmission data. In order to keep the reception buffer space to a predetermined level, a maximum number of HARQ processes are defined per TDD uplink-downlink configuration as shown in Table 3. One HARQ process is mapped to one subframe in time domain.
TABLE 3TDD UL/DLMaximum number of configurationHARQ processes04172103941251566
Referring to Table 3, if the PDSCH 301 transmitted by the eNB at subframe #0 of the ith radio frame fails to decode, the UE transmits an HARQ NACK at the subframe #7 of ith radio frame. Upon receipt of the HARQ NACK, the eNB configures the retransmission data corresponding to PDSCH 301 as PDSCH 309 and transmits the PDSCH 309 along with the PDCCH. In the exemplary case of FIG. 3, the retransmission data is transmitted in the subframe #1 of (i+1)th radio frame by taking notice that the maximum number of downlink HARQ processes is 6 in the TDD uplink-downlink configuration #6 according to the definition of Table 3. This means that there are a total of 6 downlink HARQ processes 311, 312, 313, 314, 315, and 316 between the initial transmission, i.e., PDSCH 301, and the retransmission, i.e., PDSCH 309.
In order to apply the timing relationships between a physical channel, which are specified for use in the LTE TDD system, to the LTE-A system, extra operations, in addition to the conventional timing relationships, should be defined. In more detail, there is a need for defining the timing relationship among the PDCCH, PDSCH and uplink HARQ ACK/NACK, and a method for allocating uplink HARQ ACK/NACK transmission resources for supporting self-scheduling and/or cross carrier scheduling in the situation where the TDD uplink-downlink configurations are adopted to the respective carriers aggregated differ from each other.